Receptor down-modulation. To ascertain whether increases in anti-HIV potency are accompanied by increases in capacity to sequester receptors, we tested a group of analogs in a steady-state CCR5 down-modulation assay using primary CD4-positive human lymphocytes. In contrast to the affinity determinations, the rank order of the compounds tested here, in terms of both magnitude of receptor down-modulation and its duration, is the same as their rank order for potency as HIV inhibitors (PSC-RANTES > NNY-RANTES > AOP-RANTES > Met-RANTES ≈ CAP-RANTES; Fig. 3 B and C ). These results, in agreement with other studies (20–24), suggest that the capacity of chemokine variants to induce coreceptor sequestration is a key parameter for anti-HIV potency but show that affinity plays no role.